THE METHYLTHIO GROUP (MS(2)) OF N-6-(4-HYDROXYISOPENTENYL)-2-METHYLTHIOADENOSINE (MS(2)IO(6)A) PRESENT NEXT TO THE ANTICODON CONTRIBUTES TOTHE DECODING EFFICIENCY OF THE TRANSFER-RNA

A Salmonella typhimurium LT2 mutant which harbors a mutation (miaB2508 ::Tn10dCm) that results in a Ser reduction in the activities of the am ber suppressors supF30 (tRNA(CUA)(Tyr), supD10 (tRNA(CUA)(Ser)), and s upJ60 (tRNA(CUA)(Leu)) was isolated. The mutant was deficient in the m ethylthio group (ms(2)) of N-6-(4-hydroxyisopentenyl)-2-methylthioaden osine (ms(2)io(6)A), a modified nucleoside that is normally present ne xt to the anticodon (position 37) in tRNAs that read codons that start with uridine. Consequently, the mutant had i(6)A37 instead of ms(2)io (6)A37 in its tRNA. Only small amounts of io(6)A37 was found. We sugge st that the synthesis of ms(2)io(6)A occurs in the following order: A- 37-->i(6)A37-->ms(2)i(6)A37-->ms(2)io(6)A37. The mutation miaB2508::Tn 10dCm was 60% linked to the nag gene (min 15) and 40% linked to the fu r gene and is located counterclockwise from both of these genes. The g rowth rates of the mutant in four frowth media did not significantly d eviate from those of a wild-type strain. The polypeptide chain elongat ion rate was also unaffected in the mutant. However, the miaB2508::Tn1 0dCm mutation rendered the cell more resistant or sensitive, compared with a wild-type cell, to several amino acid analogs, suggesting that this mutation influences the regulation of several amino acid biosynth etic operons. The efficiencies of the aforementioned amber suppressors were decreased to as low as 16%, depending on the suppressor and the codon context monitored, demonstrating that the ms(2) group of ms(2)io (6)A contributes to the decoding efficiency of tRNA. However, the majo r impact of the ms(2)io(6) modification in the decoding process comes from the io(6) group alone or from the combination of the ms(2) and io (6) groups, not from the ms(2) group alone.